Abstract
Background: The basic/helix-loop-helix (bHLH) transcription factor family exists in all three eukaryotic kingdoms as important participants in biological growth and development. To date, the comprehensive genomic and functional analyses of bHLH genes has not been reported in cucumber (Cucumis sativus L.). Results: Here, a total of 142 bHLH genes were identified and classified into 32 subfamilies according to the conserved motifs, phylogenetic analysis and gene structures in cucumber. The sequences of CsbHLH proteins were highly conserved based on the results of multiple sequence alignment analyses. The chromosomal distribution, synteny analysis, and gene duplications of these 142 CsbHLHs were further analysed. Many elements related to stress responsiveness and plant hormones were present in the promoter regions of CsbHLH genes based on a cis-element analysis. By comparing the phylogeny of cucumber and Arabidopsis bHLH proteins, we found that cucumber bHLH proteins were clustered into different functional clades of Arabidopsis bHLH proteins. The expression analysis of selected CsbHLHs under abiotic stresses (NaCl, ABA and low-temperature treatments) identified five CsbHLH genes that could simultaneously respond to the three abiotic stresses. Tissue-specific expression profiles of these five genes were also analysed. In addition, 35S:CsbHLH041 enhanced the tolerance to salt and ABA in transgenic Arabidopsis and in cucumber seedlings, suggesting CsbHLH041 is an important regulator in response to abiotic stresses. Lastly, the functional interoperability network among the CsbHLH proteins was analysed. Conclusion: This study provided a good foundation for further research into the functions and regulatory mechanisms of CsbHLH proteins and identified candidate genes for stress resistance in cucumber.